Vehicle headlights are designed to project high intensity beams of light in prescribed patterns to illuminate certain portions of the road in front of the vehicle. Generally, vehicles are equipped with a first pair of headlights for projecting so-called high beams of light to illuminate the highway in front of the vehicle. A second pair of headlights projects so-called low beams of light that are directed downwardly and to the right of the high beam so as not to blind an oncoming motorist. In some vehicles, high and low beam patterns are projected from the same lamps using different filaments.
In order to produce a desired illumination that maximizes visibility at night without producing excessive glare for oncoming motorists, the headlights must be properly aimed and produce a particular light pattern. Standards for headlight aiming and illumination patterns are prescribed by industrial groups, such as the Society of Automotive Engineers (SAE), and by governmental agencies. In general, different aiming and illumination pattern standards have been prescribed for different geographical areas, such as North America and Europe.
Methods for determining proper vehicle headlight aiming during vehicle manufacture and after vehicles have been in use are known. Two examples of known methods are described in U.S. Pat. No. 3,515,483 to Irwin and U.S. Pat. No. 4,435,078 to de Brabander et al. Irwin '483 and de Brabander '078 use a lens to focus a headlight light beam within a compact electro-optical unit. A number of discrete light intensity sensors are disposed within the optical system for sensing the intensity of light at a relatively small number of positions in the light beam.
In Irwin '483, which is owned by the assignee of the present invention, the intensities of light detected by each of nine discrete light intensity sensors are compared in selected pairs to determine whether established aiming criteria are satisfied within a prescribed degree of accuracy. If the accuracy is not achieved, the position of the headlight is adjusted to achieve the proper aim. This method of comparing the relative intensities of different portions of the light pattern is referred to as the fractional balance method.
In de Brabander '078, a variation of the fractional balance aiming technique is employed. There, light intensities measured at about the same number of discrete points by discrete light intensity sensors are used directly in comparison tests, or are multiplied by fractional constants to determine whether a headlight is accurately aimed or not.
The fractional balance method for headlight aiming is also described in Hopkins, et al, U.S. Pat. No. 4,948,249, which is also owned by the assignee of the present invention. In Hopkins '249, the light beam of a headlight is projected on a reflective surface or screen and includes an image sensor for sensing the light intensity in each cell of a continuous matrix of cells that overlies the reflected light beam. The image sensor produces an electrical signal, either in digital or analog form, for each cell. The signal indicates the cell location and the light intensity in that cell. Analog signals, such as are produced by a charge coupled device, are preferably digitized. The digital signals, i.e. pixels, may be manipulated by a computer to determine headlight aiming accuracy using the fractional balance method, to correct inaccurate aiming, and to describe the illumination pattern of the headlight.
Another known method for aiming vehicle headlights is commonly referred to as "hot spot aiming". Hot spot aiming detects the location of the brightest location on the headlight intensity pattern (i.e., the "hot spot"), and compares the location of the hot spot with appropriate specifications to determine if the headlight is properly aimed.
The above-described methods also provide a display of the headlight light intensity pattern as part of the aiming process. For example, Hopkins '249 displays the light intensity pattern of the headlight as a series of isocandela lines superimposed on a graphical display of an x-y axis. Alternatively, the light intensity pattern can be displayed as a simulated light beam on a simulated road surface, such as shown in Hopkins, U.S. Pat. No. 5,164,785, which is also owned by the assignee of the present invention. Hopkins '785 also discloses to alternatively display the intensity pattern of the actual headlight with the intensity pattern of a properly aimed headlight having the specifications of either the manufacturer or the regulating governmental agency.
While the above-described methods provide accurate aiming of most vehicle headlights, certain vehicle headlights currently commercially available, and some of those under development, have illumination patterns which cannot be properly aimed by one (or more) of these known methods. For example, Hopkins '249 and Hopkins '785 require certain geometrical and spacial relationships to exist in the headlight illumination pattern for proper aiming. In particular, one of the points must lie on the horizontal axis 10.5 inches to the right from the intersection of orthogonal horizontal and vertical axes and have an intensity equal to 20% of the maximum low beam intensity. The other point must lie on the vertical axis 5 inches below the intersection of the horizontal and vertical axes and have an intensity of 30% of the maximum low beam intensity. However, not all headlights have illumination patterns which have these particular geometrical and spacial criteria.
Accordingly, there is a demand in the industry for an improved headlight aiming method which provides accurate and rapid results, regardless of the type of headlight being tested.